In conventional pressure wave superchargers for internal combustion engines, a gas pocket is provided in the gas casing between each high pressure exhaust gas duct and low pressure exhaust gas duct. Part of the high pressure exhaust gas flow expelled from the engine is branched off into this gas pocket in order, in conjunction with an expansion pocket provided in the air casing, to improve the low pressure scavenging, i.e., the scavenging of the expanded exhaust gas from the rotor cells. The result of good low pressure scavenging is reduced exhaust gas recirculation, i.e., the penetration of exhaust gas into the combustion air is reduced. A large amount of exhaust gas recirculation in the idling range would adversely affect the even running of the engine.
Branching off high pressure exhaust gas into the gas pocket does, however, reduce the energy available for compressing the supercharge air. As full load, there is a wide range of speeds and temperatures within which this energy would be the desirable in order to increase the power of the engine. It would be possible to utilize this energy within this operating range if the supply of high pressure exhaust gas into the gas pocket was prevented under this condition because the low pressure scavenging is always ensured at full load. The gas pocket is therefore superfluous under this operating condition.
It follows that a gas pocket of this type with a constant inlet flow cross-section represents a compromise which accepts the fact that the energy of the high pressure exhaust gases is not used for compressing the supercharge air in the best possible way over the whole of the operating range of the engine. Shutting off the supply to the gas pocket in the lower full load range, permits better matching between the supercharge air supply from the pressure wave supercharger and the air requirements of the engine.
The present invention arose from the objective, based on the above consideration, of dividing the high pressure exhaust gas flow emerging from the engine into a main flow through the high pressure exhaust gas duct and a portion branched into the gas pocket in a relatively simple manner and in a way matched to the particular power range of the engine.
Two possible ways of feeding the gas pocket are known. The simpler consists of a narrow connecting duct between the high pressure exhaust gas duct and the gas pocket on the end surface of the gas casing facing the rotor. In this case, the static pressure in the gas pocket is that present in the main flow and this type of feed is therefore called static gas pocket feed. The second possibility is the total pressure feed in which a gas pocket duct is branched off from the high pressure exhaust gas duct into the gas pocket before the latter duct enters the rotor space. The gas pocket duct is then located in such a way that the gas flow branched off is only slightly deflected relative to the direction of the main flow. As a result, the dynamic pressure of the gas velocity is also effective in the gas pocket in addition to the static pressure. In a device of this type with total pressure feed, known from EP-PS No. 0 039 375, an attempt is made to control the supply to the gas pocket, and hence the division of the high pressure exhaust gas flow, by means of a bimetal flap. This is clamped with one end in the gas casing at the beginning of the gas pocket supply duct and permits completely free supply to the gas pocket at room temperature. During operation, the bimetal flap bends as a function of the exhaust gas temperature in such a way that the gas pocket supply is initially only slightly reduced with increasing temperature, for the purpose of good low pressure scavenging. The supply cross-section becomes gradually smaller with increasing exhaust gas temperature and finally, in the upper load range, is fully closed in order to make as much exhaust gas energy as possible available for compressing the supercharge air.
This device does not, however, permit the flap position to be controlled in an ideal manner, such as that demanded by the engine as a function of the particular operating condition, because the behavior of the flap metal cannot be matched reliably to the particular temperature. In addition, the flap may be subject to grain structure changes which, after longer operating periods, change the curvature as a function of the temperature. A further fault is the delayed response of the flap deformation to changes in temperature, which again prevents the desired coordination between the flap adjustment and the operating condition. Another particular disadvantage, however, is that such a device cannot control the flap position as a function of the supercharge pressure by means of a characteristic stored in a microprocessor. It is only such a control system, however, which permits optimum matching between the supply to the gas pocket and the particular operating condition of the engine.
The present invention arose from the requirement for such a device, preferably controlled by characteristic curves, for matching the total pressure feed to the operating condition of the engine. Such a control device ensures that the supercharge air flow of the pressure wave supercharger approximates as well as possible to the maximum over the whole operating range of the engine.
In addition to the controllable division, already mentioned, of the high pressure exhaust gas flow into a main flow for compressing the supercharge air and into a gas pocket flow for improving the low pressure scavenging, the invention also has the objective of making a change in the degree of recirculation, i.e., the proportion of exhaust gas penetrating into the supercharge air. This is accomplished by means of a special embodiment, in order to ensure the observance of limiting values of oxides of nitrogen which may possibly be required by law.
By appropriate dimensioning and control of the supply to the gas pocket, it is also possible, using a special variant, to reduce the maximum supercharge pressure or the maximum pressure ratio to the allowable maximum value so that a separate wastegate, which is the blow-down valve for excessive supercharge pressure, becomes unnecessary.